1. Field of the Invention
The invention relates to a rotary encoder and, more particularly, to a rotary encoder suitable for measuring a rotational speed, a rotational displacement, or the like of a rotary object such that when a laser beam or a light beam from a laser diode, an LED, or the like enters a radial diffraction grating of a disk attached to a rotary object (scale), a diffraction light of a predetermined order from the radial diffraction grating is subjected to a phase modulating function according to the rotational speed or rotational direction of the disk.
2. Related Background Art
Hitherto, there is a rotary encoder as a measuring instrument which can measure rotational information such as rotational amount, rotating direction, or the like of a rotary object in an NC machine tool or the like at a high precision, for example, on a submicron unit basis. Such a rotary encoder is used in various fields.
Particularly, as a rotary encoder of a high precision and a high resolution, there has widely been known a rotary encoder of the diffraction light interference system such that a coherent light beam such as a laser beam or the like enters a diffraction grating provided for a moving object, diffraction lights of predetermined orders generated from the diffraction grating are mutually interfered, and the number of bright and dark portions of the resultant interference fringe is counted,. thereby obtaining a moving state such as movement amount, movement information, or the like of the moving object.
In such a kind of rotary encoder of the diffraction light interference system, when realizing a high resolution and a high precision by using fine gratings (radial gratings), only the diffraction lights of specific orders are extracted by the optical system among a number of diffraction lights generated from the fine gratings and optical paths are overlapped by proper optical means, thereby obtaining an interference signal.
In the rotary encoder, the following conditions are generally required.
(1-a) By using a disk (rotary disk) on which radial gratings of small diameters have been recorded at a high density, a high resolution and a low inertia are obtained.
(1-b) The whole apparatus is thin and small.
(1-c) The encoder is of the unit type such that a disk and a detecting head and the like can be separated and directly assembled into an object to be measured, and when they are assembled, they can be easily handled.
On the other hand, in European Patent Publication No. 0565056, the applicant of the present invention has already proposed a linear encoder in which diffraction light beams which are reflected and diffracted from a scale are properly interfered, thereby miniaturizing the entire apparatus.